The 450 Ackley Magnum was designed by Parker Ackley around 1960. This cartridge is based on a full length (2,85-inch) H&H; belted magnum case, necked down to 45-caliber. Compared to the tapered-case 458 Watts, this Ackley-designed number does achieve the same velocity with somewhat less pressure. Conversely, this cartridge does provide for slightly more muzzle velocity when loaded to any given peak chamber pressure. 

This text is based on information from “Cartridges of the World”, Hodgdon reloading manual, the cartridge designer and/or own resources.